1. Field of the Invention
The present invention relates to an external modulator using an external modulating device of an interference type and a method for externally modulating light.
2. Description of the Related Art
An external modulating device of an interference type is used to change intensity of input light in accordance with a level of a modulation signal (electrical signal). Therefore, an optical signal can be generated by the external modulating device.
The above modulation signal is formed by superimposing a DC voltage (hereinafter called as bias voltage) on a main signal having data to be transmitted by the optical signal. The external modulating device has a characteristic in which an operation point is changed in accordance with the level of the bias voltage to be applied. Then, the bias voltage superimposed on the modulation signal is arbitrarily adjusted, so that the operation point can be arbitrarily adjusted.
However, the external modulating device has a characteristic in which the operation point is varied in accordance with a change of temperature even if the bias voltage is set to be constant. Due to this, it is necessary to adjust the operation point to an operation stable point in which such a variation of the operation point is compensated and a suitable modulation can be performed. For that object, under a condition that the bias voltage is variable the bias voltage is adjusted by, for example, a lock-in amplifier such that the operation point always becomes the operation stable point.
The external modulating device also has a characteristic in which the change of intensity of an output light, is caused in accordance with increase in the bias voltage, becomes a periodic function. More specifically, for example, the relationship between intensity of output light of the external modulating device and the bias voltage can be shown as in FIG. 1.
Moreover, the external modulating device has a characteristic in which a DC drift is provided. The DC drift is a phenomenon in which the level of the bias voltage, which is necessary for obtaining a certain intensity of output light, is changed with the passage of time. More specifically, if the relationship between intensity of output light in its initial state and the bias voltage is in a state as shown by a solid line of FIG. 1, the relationship therebetween is changed to a state as shown by a broken line of FIG. 1 after a certain period time is passed. Thereby, in a case where the bias voltage, which can obtain the maximum intensity of output light in its initial state, is Va, the bias voltage, which can obtain the same intensity of output light after a certain period of time is passed, is changed to Vb. Therefore, the bias voltage for adjusting the operation point to the operation stable point, is changed even by the DC drift. However, by providing the above-mentioned adjustment of the bias voltage, influence of the DC drift can be compensated and the operation point can be adjusted to the operation stable point.
However, unlike the variation of the operation point, which is caused by the change of temperature, in the DC drift, there is a tendency that the bias voltage is increased in a direction of a positive voltage if the bias voltage is positive (one direction). In other words, if a difference between the bias voltage, which can obtain certain intensity of output light in its initial state, and the bias voltage, which can obtain certain intensity of output light after a certain period of time is passed, is set to an amount of drift (for example, Va-Vb of FIG. 1), the relationship between passing time from the initial state and the amount of drift can be shown in FIG. 2. The larger the bias voltage becomes, the larger the amount of drift for a fixed period of time becomes.
Then, in order to compensate for influence of the DC drift, the bias voltage must be continued to be increased or decreased. However, it is difficult to change the bias voltage limitlessly, and the variable range of the bias voltage is limited by a power supply voltage. Due to this, if the bias voltage changes to the limit of the variable range, the controlling is saturated. Then, the following compensation for influence of the DC drift cannot performed, and the life of an apparatus using the external modulating device is completed in the point of the continuous operation.
In the external modulating device, the change of intensity of output light, is caused in accordance with increase in the bias voltage, becomes the periodic function as shown in FIG. 1, there exists a plurality of operation stable points as shown in FIG. 1. Then, in the initial state after power supply, it is not fixed to which operation stable point the operation point is adjusted. Due to this, in the initial state after power supply, if the operation is stabilized at an operation stable point, which is achieved by the bias voltage close to an upper or lower limit of the variable range, the bias voltage is set to be in the vicinity of the upper or lower limit of the variable range. Due to this, a dynamic range of control becomes narrow, and time till the controlling is saturated (life of continuous operation) becomes short. More specifically, for example, as shown in FIG. 3, in a case A in which the bias voltage is set to be close to the upper limit of the variable range at the time of power supply, the life of the continuous operation becomes considerably short as compared with a case B in which the bias voltage is set to be close to the center of the variable range.